In a guardrail wherein a horizontal downwardly open metal channel is filled with polyurethane foam (see the aforementioned applications), sockets are formed in the foam mass in the channel and vertical posts are received in these pockets, spaced apart by at least 10 meters. In this arrangement it is necessary to galvanize or otherwise protect the exposed surfaces of the metal channel from corrosion. In another known arrangement, heavy cables are tightly stretched between posts. The cables are covered with synthetic-resin bodies which protect the cable and cushion shocks against it. Such an arrangement is difficult to set up, and offers only nominal resistance to deflection.